We shall study how isoniazid (INH) inhibits the synthesis of mycolic acids. The biosyntheses of mycolic acids is thought to involve the elongation reactions (to synthesize a C26 and a C54 fatty acid) and the condensation reaction where the two fatty acids condense to form the mycolic acid. We shall seek the postulated C54 fatty acid, isolate and characterize the lipid by IR, NMR and mass spectrometry. We shall study the effect of INH on its synthesis at both whole cell and cell-free levels. Finally the condensation reaction will be studied by utilizing a simple model system present in the Corynebacterium diphtheriae. We shall relate the INH inhibition of mycolate synthetase (MS) activity to the primary mode of action of the drug. Three approaches will be taken: (a) isolate an INH uptake competent, INH resistant strain of mycobacteria, and determine the sensitivity of the MS enzyme system to INH; (b) when the site of action of INH on the MS activity is determined, add the appropriate fatty acids to the culture medium to overcome this block and reverse the INH effects; (c) examine known inhibitors of fatty acid elongation reactions (trichloroacetic acid, thiocarbamates and mercaptoethanol) for their action on growth, viability and the MS activity in mycobacteria.